280 research outputs found
Long-term monitoring in IC4665: Fast rotation and weak variability in very low mass objects
We present the combined results of three photometric monitoring campaigns
targeting very low mass (VLM) stars and brown dwarfs in the young open cluster
IC4665 (age ~40 Myr). In all three runs, we observe ~100 cluster members,
allowing us for the first time to put limits on the evolution of spots and
magnetic activity in fully convective objects on timescales of a few years. For
20 objects covering masses from 0.05 to 0.5 Msol we detect a periodic flux
modulation, indicating the presence of magnetic spots co-rotating with the
objects. The detection rate of photometric periods (~20%) is significantly
lower than in solar-mass stars at the same age, which points to a mass
dependence in the spot properties. With two exceptions, none of the objects
exhibit variability and thus spot activity in more than one season. This is
contrary to what is seen in solar-mass stars and indicates that spot
configurations capable of producing photometric modulations occur relatively
rarely and are transient in VLM objects. The rotation periods derived in this
paper range from 3 to 30h, arguing for a lack of slow rotators among VLM
objects. The periods fit into a rotational evolution scenario with pre-main
sequence contraction and moderate (40-50%) angular momentum losses due to wind
braking. By combining our findings with literature results, we identify two
regimes of rotational and magnetic properties, called C- and I-sequence. Main
properties on the C-sequence are fast rotation, weak wind braking, Halpha
emission, and saturated activity levels, while the I-sequence is characterised
by slow rotation, strong wind braking, no Halpha emission, and linear
activity-rotation relationship. Rotation rate and stellar mass are the primary
parameters that determine in which regime an object is found. (abridged)Comment: 16 pages, 10 figures, accepted for publication in MNRA
Wer das Geld hat, hat die Macht? : Verhandlungen von Property Gaps in Paarwelten
Während für Paare eine zunehmende Verbreitung partnerschaftlicher Beziehungsnormen konstatiert wird, herrschen nach wie vor Geschlechterungleichheiten, die sich u.a. im gender wage gap oder gender care gap dokumentieren. Entsprechend wurden bislang Ungleichheiten bei Paaren primär anhand von Einkommensdifferenzen oder entlang der innerpartnerschaftlichen Verteilung von bezahlter Erwerbs- und unbezahlter Fürsorgearbeit untersucht. Es fehlen Analysen zu den ungleichheits(re)produzierenden Mechanismen der Eigentumsverteilung und -aushandlung innerhalb von Paarbeziehungen, denn Eigentum geht über Einkommen hinaus und umfasst Vermögen, Immobilien, Sachgüter, aber auch Schulden als Form negativen Eigentums. Der Beitrag fokussiert ausgehend von Interviewmaterial aus dem Teilprojekt B06 des SFB/TR 294 "Strukturwandel des Eigentums" auf den paarinternen Umgang mit ungleichen Eigentumsausstattungen (property gaps). Mittels eines Fallvergleichs werden Strategien und Effekte der Umverteilung und Vergemeinschaftung von Eigentum zur Überbrückung von property gaps reflektiert. Es zeigt sich, dass in der Analyse des paarinternen doing property die Differenzierung zwischen Eigentumsströmen (flows) und Eigentumsbeständen (stocks) Berücksichtigung finden muss. Zudem fungiert der individuelle Erfahrungshintergrund der Klassenherkunft – neben Geschlecht – als Regulativ bei den meist subtil ablaufenden Eigentumsverhandlungen in Paarbeziehungen. Insgesamt wird für eine stärkere Berücksichtigung der Eigentumsdimension in der Geschlechterforschung plädiert
Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs
We present spectra of 59 nearby stars candidates, M dwarfs and white dwarfs,
previously identified using high proper motion catalogues and the DENIS
database. We review the existing spectral classification schemes and
spectroscopic parallax calibrations in the near-infrared -band and derive
spectral types and distances of the nearby candidates. 42 stars have
spectroscopic distances smaller than 25 pc, three of them being white dwarfs.
Two targets lie within 10 pc, one M8 star at 10.0 pc (APMPM J0103-3738), and
one M4 star at 8.3 pc (LP 225-57). One star, LHS 73, is found to be among the
few subdwarfs lying within 20 pc. Furthermore, together with LHS 72, it
probably belongs to the closest pair of subdwarfs we know.Comment: 10 pages, 7 figures, accepted for publication in MNRA
About the nature of Mercer14
We used UKIRT near infrared (NIR) broad band JHK photometry, narrow band
imaging of the 1-0S(1) molecular hydrogen emission line and mid infrared
Spitzer IRAC data to investigate the nature of the young cluster Mercer14.
Foreground star counts in decontaminated NIR photometry and a comparison with
the Besancon Galaxy Model are performed to estimate the cluster distance. This
method yields a distance of 2.5kpc with an uncertainty of about 10% and can be
applied to other young and embedded clusters. Mercer14 shows clear signs of
ongoing star formation with several detected molecular hydrogen outflows, a
high fraction of infrared excess sources and an association to a small gas and
dust cloud. Hence, the cluster is less than 4Myrs old and has a line of sight
extinction of A_K=0.8mag. Based on the most massive cluster members we find
that Mercer14 is an intermediate mass cluster with about 500Mo.Comment: 10pages, 2tables, 5figures, accepted for publication by MNRAS, a
version with higher resolution figures can be found at
http://astro.kent.ac.uk/~df
Rotation and variability of very low mass stars and brown dwarfs near Epsilon Ori
We explore the rotation and activity of very low mass (VLM) objects by means
of a photometric variability study. Our targets in the vicinity of Epsilon Ori
belong to the OriOB1b population in the Orion star-forming complex. In this
region we selected 143 VLM stars and brown dwarfs (BDs), whose photometry in
RIJHK is consistent with membership of the young population. The variability of
these objects was investigated using a densely sampled I-band time series
covering four consecutive nights with altogether 129 data points per object.
Our targets show three types of variability: Thirty objects, including nine
BDs, show significant photometric periods, ranging from 4h up to 100h, which we
interpret as the rotation periods. Five objects, including two BDs, exhibit
variability with high amplitudes up to 1 mag which is at least partly
irregular. This behaviour is most likely caused by ongoing accretion and
confirms that VLM objects undergo a T Tauri phase similar to solar-mass stars.
Finally, one VLM star shows a strong flare event of 0.3 mag amplitude. The
rotation periods show dependence on mass, i.e. the average period decreases
with decreasing object mass, consistent with previously found mass-period
relationships in younger and older clusters. The period distribution of BDs
extends down to the breakup period, where centrifugal and gravitational forces
are balanced. Combining our BD periods with literature data, we found that the
lower period limit for substellar objects lies between 2h and 4h, more or less
independent of age. Contrary to stars, these fast rotating BDs seem to evolve
at constant rotation period from ages of 3 Myr to 1 Gyr, in spite of the
contraction process. Thus, they should experience strong rotational braking.Comment: 17 pages, 13 figures, A&A, in pres
Impact of Record-Linkage Errors in Covid-19 Vaccine-Safety Analyses using German Health-Care Data: A Simulation Study
With unprecedented speed, 192,248,678 doses of Covid-19 vaccines were
administered in Germany by July 11, 2023 to combat the pandemic. Limitations of
clinical trials imply that the safety profile of these vaccines is not fully
known before marketing. However, routine health-care data can help address
these issues. Despite the high proportion of insured people, the analysis of
vaccination-related data is challenging in Germany. Generally, the Covid-19
vaccination status and other health-care data are stored in separate databases,
without persistent and database-independent person identifiers. Error-prone
record-linkage techniques must be used to merge these databases. Our aim was to
quantify the impact of record-linkage errors on the power and bias of different
analysis methods designed to assess Covid-19 vaccine safety when using German
health-care data with a Monte-Carlo simulation study. We used a discrete-time
simulation and empirical data to generate realistic data with varying amounts
of record-linkage errors. Afterwards, we analysed this data using a Cox model
and the self-controlled case series (SCCS) method. Realistic proportions of
random linkage errors only had little effect on the power of either method. The
SCCS method produced unbiased results even with a high percentage of linkage
errors, while the Cox model underestimated the true effect
Very low-mass members of the Lupus 3 cloud
We report on a multi-band survey for very low-mass stars and brown dwarfs in
the Lupus 3 cloud with the Wide Field Imager (WFI) at the ESO/MPG 2.2m
telescope on La Silla Observatory (Chile). Our multiband optical photometry is
combined with available 2MASS JHK photometry to identify 19 new young stars and
3 brown dwarf candidates as probable members of this star forming region. Our
objects are mostly clustered around the cloud core. Stars and brown dwarfs have
similar levels of H-alpha emission, probably a signature of accretion. One
object, a brown dwarf candidate, exhibits a near-infrared excess, which may
indicate the presence of a disk, but its H-alpha emission cannot be confirmed
due to its faintness in the optical passbands. We also find two visual pairs of
probable Lupus 3 members that may be wide binaries
SSPMJ1102-3431 brown dwarf characterization from accurate proper motion and trigonometric parallax
In 2005, Scholz and collaborators (Scholz et al. 2005) discovered, in a
proper motion survey, a young brown dwarf SSSPMJ1102-3431(SSSPMJ1102) of
spectral type M8.5, probable member of the TW Hydrae Association (TWA) and
possible companion of the T Tauri star TW Hya. The physical characterization of
SSSPMJ1102 was based on the hypothesis that it forms a binary system with TW
Hya. The recent discovery of a probable giant planet inside the TW Hya
protoplanetary disk with a very short-period (Setiawan et al. 2008) and a disk
around SSSPMJ1102 (Riaz and Gizis 2008) make it especially interesting and
important to measure well the physical parameters of SSSPMJ1102. Trigonometric
parallax and proper motion measurements of SSSPMJ1102 are necessary to test for
TWA membership and, thus, to determine the mass and age of this young brown
dwarf and the possibility that it forms a wide binary system with TW Hya. Two
years of regular observations at the ESO NTT/SUSI2 telescope, have enabled us
to determine the trigonometric parallax and proper motion of SSSPMJ1102. Our
parallax and proper motion determination allow us to precisely describe the
physical properties of this low mass object and to confirm its TWA membership.
Our results are not incompatible with the hypothesis that SSSPMJ1102 is a
binary companion of the star TW Hya.Comment: accepted for publication in A&
Rotation and accretion of very low mass objects in the SigmaOri cluster
We report on two photometric monitoring campaigns of Very Low Mass (VLM)
objects in the young open cluster around SigmaOrionis. Our targets were
pre-selected with multi-filter photometry in a field of 0.36 sqdeg. For 23 of
these objects, spanning a mass range from 0.03 to 0.7 MS, we detect periodic
variability. Of these, 16 exhibit low-level variability, with amplitudes of
less than 0.2 mag in the I-band, which is mostly well-approximated by a sine
wave. These periodicities are probably caused by photospheric spots co-rotating
with the objects. In contrast, the remaining variable targets show high-level
variability with amplitudes ranging from 0.25 to 1.1 mag, consisting of a
periodic light variation onto which short-term fluctuations are superimposed.
This variability pattern is very similar to the photometric behaviour of
solar-mass, classical T Tauri stars. Low-resolution spectra of a few of these
objects reveal strong Halpha and Ca-triplet emission, indicative of ongoing
accretion processes. This suggests that 5-7% of our targets still possess a
circumstellar disk. In combination with previous results for younger objects,
this translates into a disk lifetime of 3-4 Myr, significantly shorter than for
solar mass stars. The highly variable objects rotate on average slower than the
low-amplitude variables, which is expected in terms of a disk-locking scenario.
There is a trend towards faster rotation with decreasing mass, which might be
caused by shortening of the disk lifetimes or attenuation of magnetic fields.Comment: 19 pages, 14 figures, A&A, in pres
The brown dwarf population in the Chamaeleon I cloud
We present the results of a multiband survey for brown dwarfs in the
Chamaeleon I dark cloud with the Wide Field Imager (WFI) camera at the ESO/MPG
2.2-m telescope on La Silla (Chile). The survey has revealed a substantial
population of brown dwarfs in this southern star forming region. Candidates
were selected from R, I and H-alpha imaging observations. We also observed in
two medium-band filters, M855 and M915, for the purpose of spectral type
determination. The former filter covers a wavelength range containing spectral
features characteristic of M-dwarfs, while the latter lies in a relatively
featureless wavelength region for these late-type objects. A correlation was
found between spectral type and (M855-M915) colour index for mid- to late
M-type objects and early L-type dwarfs. With this method, we identify most of
our object candidates as being of spectral type M5 or later. Our results show
that there is no strong drop in the number of objects for the latest spectral
types, hence brown dwarfs may be as abundant as low-mass stars in this region.
Also, both kind of objects have a similar spatial distribution. We derive an
index of the mass function in this region of dispersed
star formation, in good agreement with the values obtained in other star
forming regions and young clusters. Some of the brown dwarfs have strong
H-alpha emission, suggesting mass accretion. For objects with published
infrared photometry, we find that strong H-alpha emission is related to a
mid-infrared excess, indicative of the existence of a circumstellar disk.Comment: Accepted for publication in Astronomy & Astrophysic
- …